The invention relates generally to resource allocation in a wireless code division multiple access (CDMA) communication system. More specifically, the invention relates to assigning uplink scrambling codes in a CDMA communication system.
FIG. 1 depicts a wireless spread spectrum Code Division Multiple Access (CDMA) communication system 20. A base station 22 communicates with user equipment (UE) 241–24n in its operating area. In a spread spectrum CDMA system 20, data signals are communicated between UEs 241–24n and the base station 22 over the same spectrum. Each data signal in the shared spectrum is spread with a unique chip code sequence. Upon reception, using a replica of the original chip code sequence, a particular data signal is recovered.
Since signals are distinguished by their chip code sequences (code), separate dedicated communication channels are created using different codes. Signals from the base station 22 to the UEs 241–24n are sent on downlink channels and signals from the UEs 241–24n to the base station 22 are sent on uplink channels. For coherent detection of downlink transmissions by the UEs 241–24n, pilot signals are transmitted to all of the UEs 241–24n within the base station's operating range. The UEs 241–24n condition their receivers based on the pilot signals to enable data reception.
In many CDMA systems, a random access channel, such as the common packet channel (CPCH), is used for some uplink transmissions. A CPCH is capable of carrying packets of data from different UEs 241–24n. Each packet is distinguishable by its code. For detection by the base station 22, the packets have a preamble which also distinguishes it from other packets. The CPCH is typically used to carry infrequently communicated data at high rates.
FIG. 2 illustrates a CPCH time slot and frame structure. The CPCH structure is time divided into radio frames 301–30m having time slots 281–28n, such as eight time slots proposed for the Third Generation Mobile Telecommunications System (IMT-2000)-UMTS. A radio frame 301–30m in IMT-2000 is 10 milliseconds in duration and each time slot is 1.25 ms. The radio frames 301–30m are grouped into superframes 32. Each superframe 32 has a fixed number of radio frames 301–30m, such as 72 radio frames in IMT-2000.
To allow more than one UE 241–24n to use a given time slot 281–28n, multiple signatures are used to distinguish the UEs 241–24n. In IMT-2000, sixteen different signatures are used. A particular signature used within a particular time slot is referred to as an access opportunity. FIG. 3 illustrates the access opportunities 2611–26nm of the CPCH. For instance, as proposed for IMT-2000, for each of the 8 time slots, one out of 16 signatures is available to be chosen, resulting in 128 access opportunities. Each access opportunity 2611–26nm is preassigned an uplink scrambling code. The scrambling code is a function of the time slot TK and the signature SK that the UE used for access. Accordingly, the uplink scrambling code, CK, is a function of the time slot, TK, and signature, SK, of the access opportunity 2611–26nm as in Equation 1.CK=8*TK+SK  Equation 1The UE 241 transmits a data packet using a selected access opportunity 2611–26mn. Upon identifying a particular access opportunity 2611–26nm, the base station 20 sends out an acknowledgment message (ACK) if the corresponding scrambling code is available. The ACK message may be one of several types, such as simply being a downlink transmission of the signature associated with the UE's access attempt. If the scrambling code is not available, a negative acknowledgment (NAK) is sent. After receiving the appropriate ACK message, the UE 241 selects the proper uplink scrambling code to transmit the packet data on the CPCH. If the UE 241 receives a NAK, it will re-attempt access by transmitting another packet.
This approach for assigning uplink scrambling codes has drawbacks. A typical scrambling code is only 10 ms in length. A transmitted data packet may last more than one radio frame 30. Since a data packet may last for multiple radio frames, the scrambling code used for that packet can only be reassigned after the transmission of that packet is complete. As a result, the number of CPCH users is limited by the number of scrambling codes assigned to the access opportunities 2611–26mn, such as 128 scrambling codes. Additionally, if a second user uses the same access opportunity 2611–26mn as an already transmitting first user, the second user will receive a NAK. Repeated negative access attempts lower the efficiency of the system 20 which is undesirable.
Accordingly, it is desirable to use alternate scrambling code assignment schemes.